1. Field of the Invention
This invention relates to a magnetic bearing which rotatably supports a rotating member, utilizing a magnetic attraction force.
2. Related Art
There is known a magnetic bearing which rotatably supports a rotating member, utilizing a magnetic attraction force. Such a magnetic bearing can support the rotating member in a non-contact condition, and therefore has many advantages such as a small bearing loss, a maintenance-free design, low noises, the obviation of the need for a lubricating oil, and the capability of being used in a vacuum condition.
A conventional magnetic bearing will be described with reference to the drawings. FIG. 1 is a cross-sectional view of a magnetic bearing disclosed in Japanese Patent Unexamined Publication No. 4-171316. In this Figure, the magnetic bearing broadly comprises two members, that is, a shell 101 and a rotation shaft 102. A motor stator 103 is mounted on an inner peripheral surface of the shell 101, and a motor rotor 104 is mounted on an outer peripheral surface of the rotation shaft 102 in opposed relation to the motor stator 103, and with this construction, a rotating force is imparted to the rotation shaft 102.
A pair of electromagnetic attraction stators 105a and 105b are mounted on the inner peripheral surface of the shell 101, and are spaced from each other in an axial direction, and electromagnetic attraction rotors 106a and 106b are mounted on the outer peripheral surface of the rotation shaft 102, and are disposed in opposed relation to the stators 105a and 105b, respectively, and these stators 105a and 105b and these rotors 106a and 106b jointly form a radial bearing, utilizing an electromagnetic attraction force.
A disk-shaped thrust plate 107 is mounted on the outer periphery of the rotation shaft 102, and electromagnetic attraction stators 108 are mounted on the inner peripheral surface of the shell 101 in such a manner that the thrust plate 107 is interposed between those stators 108. The thrust plate 107 and these stators 108 jointly form a thrust bearing, utilizing an electromagnetic attraction force.
Position detection displacement sensors 109a and 109b are mounted on the inner side of the shell 101 for detecting the position of the rotation shaft 102 in a radial direction, and a position detection displacement sensor 110 is mounted on the inner side of the shell 101 for detecting the position of the rotation shaft 102 in a thrust direction. A tool 111 is adapted to be mounted on a distal end portion of the rotation shaft 102.
The rotation shaft 102 is usually made of a ferromagnetic material, and thus, care is taken so that the magnetic efficiency of the motor rotor 104 and the thrust plate 107 will not be lowered. Each of the rotors 104, 106a and 106b comprises silicon steel sheets which are fitted on the rotation shaft 102.
For reasons mentioned below, a large bearing loss develops in the radial magnetic bearing portions of the magnetic bearing of FIG. 1. Namely, the polarity of the electromagnetic poles of the radial magnetic bearing is alternating in the rotational direction, and therefore there is a possibility that a large hysteresis loss and a large eddy current loss develop in each laminate of disk-shaped sheets mounted on the rotation shaft, thereby causing heat to be generated in the rotation shaft.
A radial magnetic bearing construction, designed to avoid these losses, is disclosed, for example, in U.S. Pat. No. 4,983,870. FIG. 2(a) is a cross-sectional view of a radial magnetic bearing portion of such a conventional construction, and FIG. 2(b) is a cross-sectional view of a rotor assembly.
In FIGS. 2(a) & 2(b) rotation shaft 246 includes flux transferring members (in the form of an I-shaped rotating laminate 258) dispersed in an I-shaped rotating laminate 260. The laminate 258 is larger in radial length than the laminate 260. The laminates 258 and 260 are held in a suitable position on the rotation shaft 246 by a restraint ring 262, associated with the rotation shaft 246, and a retaining member 250. Reference numeral 220 denotes a housing, reference numeral 212 a stator, reference numeral 214 the rotor assembly, and reference numeral 218 an electromagnet.
The retaining member 250 is welded to the rotation shaft 246, and includes an annular shoulder 263 embracing an end of the laminate 260. Rotating member bands 264 are provided in opposed relation to windings of magnetic coils 240, respectively, and serve to additionally hold the rotating laminates 258 and 260.
In the construction of FIGS. 2(a) & 2(b) the polarity of electromagnetic poles of the radial magnetic bearing is not alternating in a rotational direction. An eddy current loss tends to develop because of a change of a magnetic field intensity due to a construction in which magnetic pole-existing portions and magnetic polo-nonexisting portions alternate on the peripheral surface of a rotation shaft in a circumferential direction. In the construction of FIG. 2, however, the laminate of electromagnetic steel sheets, forming a passage for an eddy current, are insulated from one another in the circumferential direction, and therefore an eddy current is hardly produced, so that a bearing loss is extremely reduced.
Even in this construction, however, a large current loss still develops. The reason for this is that a bias current for producing a bias flux must always be flowed so that this radial magnetic bearing can effectively achieve its control function.
In order to save a waste of the bias current flowed so as to produce the bias flux, Japanese Utility Model Examined Publication No. 3-43467 discloses a conventional thrust magnetic bearing in which permanent magnets are provided in electromagnets. FIG. 3 is a cross-sectional view of such a conventional magnetic bearing.
In FIG. 3, a rotating member 302 is attracted upward or downward by an attraction force produced by an upper permanent magnet 305a or a lower permanent magnet 305b. In order to eliminate this imbalance force, the position of the rotating member is detected by a position detector 310, and a control current, corresponding to this position, is fed to coils 307a and 307b through a position controlling device 311 and an amplifier 312. As a result, the rotating member is held in a predetermined position in a thrust direction.
Yokes 304a and 304b each having annular teeth are disposed coaxially with each other, and therefore when the rotating member 302 is displaced in a radial direction, there is produced a magnetic restoring force tending to restore this rotating member into its original position where the rotating member is coaxial or concentric with a fixed member 301. Thus, the rotating member can be supported or borne in the thrust and radial directions.
In this conventional construction, since a bias flux is supplied by the permanent magnets 305a and 305b, the magnetic field polarity of each bearing surface of the rotation shaft, opposed to the thrust magnetic pole surface, is constant, and therefore an eddy current will not be produced in so far as the rotation shaft is displaced in the axial direction. However, this construction can not be directly applied, for example, to the radial magnetic bearing of FIG. 12.